The use of reinforced plastics or composites in the fabrication of structural components has grown substantially in recent years. Composite structures are formed by overlapping layers of a "towpreg", i.e., reinforcement material such as graphite fiber impregnated with a matrix material such as epoxy. Composite structures have become increasingly popular as a replacement for metallic parts, particularly in high performance applications such as in the aircraft industry, because of the high strength to weight ratio, good corrosion resistance, good impact resistance, and high electrical and thermal resistance exhibited by composite parts.
One aspect of the composites industry which has restricted the use of composite parts in some applications is that traditionally many composite parts had to be fabricated by hand or with several manual operations. The technology of automating the formation of composite parts continues to evolve, but limitations still exist particularly in the formation of parts having a relatively complex shape, i.e., parts having contoured or arcuate surfaces as opposed to cylindrical or other standard shapes.
Early attempts to automate the formation of composite parts involved the use of filament winding machines employing a wet winding technique in which fibers of filamentary material are drawn through a resin bath mounted on a traversing carriage having a pay-out eye. A form or tool, carried on a rotating mounting structure, is located with respect to the carriage such that the resin impregnated fibers are guided under tension by the pay-out eye longitudinally along the rotating tool. The pay-out eye traverses the tool from end to end laying down successive layers of fibers until the desired wall thickness is built up on the tool. The resin or matrix material is cured on the tool, and then the tool is removed leaving the cured, wound composite structure. See, for example, U.S. Pat. Nos. 378,427; 3,146,926 and 3,363,849.
One advantage of filament winding machines is that the pay-out eye can be oriented with respect to the tool such that the fibers are laid down at various angles relative to the longitudinal axis of the tool. This permits the formation of a finished composite part in which the several layers of fibers forming the wall of the part are oriented in the direction in which the part will be loaded, thus providing maximum strength with minimum wall thickness. Despite this and other advantages, a number of problems or limitations are presented by current filament winding techniques. For example, in the formation of cylindrical-shaped objects, the continuous fibers traverse the tool longitudinally from end to end to form the individual layers of the wall of the part. This produces a buildup of fibers at the ends of the part, compared to the center section thereof, which wastes fiber material at the ends of the part if it is not needed there.
Another problem with conventional filament winding machines relates to "compaction pressure", i.e., the pressure with which the fibers are applied onto the surface of a tool. The fibers are guided through a pay-out eye in filament winding machines and are applied to the surface of the tool under tension. The compaction pressure is dependent upon the tension on the fiber, the curvature of the surface of a tool and the width of the fibers. Tools having complex shapes such as arcuate or contoured surfaces with "peaks and valleys", i.e., concave and convex areas located adjacent one another along the winding axis, present problems for filament winding machines because the tension wound fibers span the concave surface adjacent a convex area. This is because no means are provided to press or compact the fibers directly into the concave area. The lack of direct compaction pressure between the fibers and tool surface in filament winding machines also creates problems in the winding of box-shaped parts. Because the compaction pressure is dependent, in part, on the curvature of the tool surface, the fibers are laid down on the flat sides of the box with little or no compaction whereas the corners of the box are highly compacted. The box-shaped part is thus unevenly compacted by filament winding machines, and has a thinner wall thickness around the corners than the sides.
The problem of automatically forming more complex composite parts has been solved to some extent by tape laying machines such as disclosed, for example, in U.S. Pat. Nos. 3,616,078; 4,822,444; 4,273,601; 3,775,219; 4,292,108; and 4,419,170. Machines of this type lay down a relatively wide "tape" which is essentially a pre-impregnated group of continuous individual fibers oriented parallel to one another on a carrier material. These tapes are carried in a placement head supported by structure capable of manipulating the placement head relative to a tool or form about a number of axes. Unlike filament winding devices, tape laying machines are capable of accommodating more complex-shaped parts because the fibers in the tape are pressed or compacted directly onto the tool by a compaction roller or shoe carried on the placement head. The mechanisms which carry the placement head are effective to maintain the roller or shoe substantially perpendicular to the surface of the tool such that the tape is pressed against non-planar surfaces of the tool. As a result, tape laying machines are more versatile than filament winding apparatus for large, gently contoured parts and have been effective in automating the production of some parts which had previously been constructed entirely by hand or with a number of hand lay up operations.
While tape laying machines have provided an advance in the fabrication of composite parts, such machines also have limitations. One problem involves an unwanted buildup of composite layers at the small ends of a tapered tool and similar parts. There is no provision in tape laying machines for decreasing the numbers of fibers within the tape as the placement head reaches the smaller ends of a tapered tool, for example, and therefore more fiber material can be built up on the ends than the center of the tool.
Another problem with tape laying machines is that they are incapable of laying down the tape along an arcuate or curved path except where the arc or angle of the path is extremely large. As mentioned above, the tape consists of fibers oriented parallel to one another on a carrier material. If the placement head of the tape laying machine is moved in an arcuate path, the tape tends to wrinkle or buckle because all of the fibers in the tape are of the same length. In order for a tape laying machine to accommodate arcuate paths, the fibers along one edge of the tape must subtend a different length than those on the opposite edge so that the tape conforms to such an arcuate path. Variation in the length of the fibers within the tape is not possible in currently available tape laying machines.
A third generation of automated devices for the fabrication of composite parts is disclosed, for example, in U.S. Pat. No. 4,699,683 to McCowin. Apparatus of the type disclosed in the McCowin patent are referred to as "fiber placement" machines and differ from tape laying machines in that they apply a number of individual fibers or tows side-by-side onto a form or tool rather than a pre-formed tape that is reeled with a carrier material. Fiber placement machines include a creel assembly consisting of a number of spools of pre-impregnated fibers, known as towpregs, which are individually fed at independently controlled rates to a fiber placement head. The fiber placement head includes structure for handling each tow individually. This structure is effective to feed the several tows side-by-side to form a fiber band which is pressed onto the surface of the tool by a compaction roller or shoe. The fiber placement head also includes structure for individually cutting one or more of the tows so that they can be "dropped off" from the remaining tows being applied to the tool.
The ability to selectively cut individual tows within the fiber band has a number of advantages. One advantage of selectively cutting individual tows is that the fiber placement head can lay down the tows in an arcuate path. This is because the length of the individual tows can vary since each individual tow is allowed to subtend a different line length compared to adjacent tows forming the fiber band. Another advantage of cutting individual tows is that material savings are obtained in forming tapered parts and the like wherein one or more of the fibers can be "dropped off" or cut as the fiber placement head reaches the ends of the tool to avoid unwanted buildup of fiber thereat. A still further advantage of permitting cutting of each tow individually is that "windows", e.g., holes, cut-outs, etc., formed in the tool can be accommodated by dropping off one or more tows as the fiber placement head moves past so that the windows are uncovered or free of fiber material.
The apparatus disclosed in the McCowin U.S. Pat. No. 4,699,683 provides distinct improvements over filament winding apparatus and tape laying machines because of its capability of individually feeding and cutting the separate fibers which form the fiber band. But the McCowin apparatus may be deficient in certain applications due to several design aspects of such apparatus.
One problem with the apparatus disclosed in McCowin U.S. Pat. No. 4,699,633 involves the location of the creel assembly which supplies the individual tows. In the McCowin apparatus, the creel assembly is carried by the fiber placement head, i.e., it is mounted directly on the fiber placement head, and the tows are unwound from each spool and separately fed beneath the compaction roller or shoe for placement onto the tool. The problem with this design is that the creel assembly is bulky and adds substantial weight to the fiber placement head, the compaction roller or shoe of which must be maintained substantially perpendicular to the surface of the tool. A tool having a relatively complex shape requires substantial movement of the fiber placement head and this is made difficult by the added weight of the creel assembly, particularly at relatively high operating speeds.
Another problem with the apparatus disclosed in the McCowin U.S. Pat. No. 4,699,683 is that a relatively complicated and cumbersome gantry system is provided to move the fiber placement head with respect to a tool. Two spaced, horizontally extending beams, each supported on vertical legs, mount a carriage movable along the longitudinal axis of such beams. A vertical support is mounted to this carriage which, at its lower end, supports a rotary plate carrying the fiber placement head. All of these structures are large and require relatively complicated mechanisms to effect movement of the fiber placement head therealong, and the overall height of the apparatus is substantial which precludes its use in many manufacturing facilities. Additionally, the size of a tool to be laid up by the McCowin apparatus is limited in width dimension to the spacing between the horizontal beams of the fiber placement head support structure. That is, the motion of the fiber placement head in a direction perpendicular to the horizontal support beams is limited by the spacing therebetween. This may present problems in certain applications wherein the tool has a large width and length dimension.